In some lubrication applications, it is difficult to get a mechanically driven pump in a remote location of a gearbox, transmission, etc. As a result, the designer has to resort to a hydraulically driven pump or an electrically driven pump. A hydraulically driven pump (i.e. a pump driven by a hydraulic motor) may be the most desirable for reliability if a source of high pressure flow is available to drive the hydraulic motor.
In many cases and as illustrated in FIG. 1, a remotely driven pump assembly 20 includes a hydraulic motor 22 (e.g. a spur gear, internal crescent gear, gerotor, or piston motor) which drives a shaft 24 to drive a low pressure pump 26 (e.g. a gear, gerotor, or piston pump). The pump draws fluid from the sump 26 for supply by the pump to the load, or for a scavenge pump to pull fluid from a sump to return to another tank, for example.
The foregoing system requires a source of relatively constant high pressure for desired operation of the pump assembly 20. In many cases, the prime mover that drives the supply pressure pump varies in speed, as in the case of a vehicle where the pump is driven off the vehicle engine and thus has a rotational speed that increases and decreases along with the engine speed. This situation is typical of automobile engine oil pumps and automatic transmission oil pumps. In these cases the pump size typically is based on the worst case, that typically being high engine idle. Consequently, in many cases the pump will start to cavitate well under the maximum speed of the engine (or other prime mover). The cavitation generates noise that can be annoying to the occupant or occupants of the vehicle or someone in the vicinity of the pump.
The simplest solution to this problem is simply to live with the noise. Others have tried jet pumps, but this approach is very inefficient.